VALLEY REGION ; CHARACTERS OF THE ROCKS. 151 
blue limestone, occur near the base of the Knox Dolomite, 
and are very closely related to similar beds of the Shale di- 
vision already described. On the other hand, the siliceous 
matter in the upper part of the formation is usually found 
in masses of chert of concretionary origin impregnating the 
dolomite, and on the breaking down of these rocks under 
the action of the weather, the calcareous parts are leached 
out while the siliceous parts remain usually in the form of 
angular flinty gravel, which forms the very characteristic 
ridges of the Knox Dolomite. In the region covered by this 
map, we have found it convenient to distinguish the area 
underlaid by the lower and more calcareous part of the 
formation and that formed by the upper or more siliceous 
part. In the former, the weathering of the limestones and 
dolomites has given rise to the formation of gently undulat- 
ing terranes with a deep red-colored sandy loam soil of 
more than average fertility, which is the base of the best 
farming lands in all these valleys. The red lands about 
Elyton, and in parts of Birmingham, and in the Alexandria 
Valley across the Coosa, are good examples. In the upper 
part of the Dolomite the cherty or siliceous matter is more 
abundant as a surface material than the calcareous, and the 
country is broken or ridgy, rather than undulating. Some 
of these flint ridges extend for long distances unbroken. 
Good examples are the ridges of the North and South High- 
lands about Birmingham. In fact this angular cherty gravel 
is found upon all the lands made by the Knox Dolomite, 
but is much more abundant and characteristic in the upper 
part. The Knox Dolomite contains very few fossils, and 
these belong to the Lower Silurian horizon of the paleon- 
tologists, but we have in the chert itself a characteristic by 
which we can as a rule distinguish it from the chert of other 
formations, that is, we find in most of it small angular cavi- 
ties of clearly defined shape which are usually thought to 
mark the places once occupied by rhombohedral crystals of 
dolomite, subsequently dissolved out. Prof. Safford was 
the first to call attention to this mark, which we have found 
to be an extremely useful one. The Knox Dolomite as well 
as the upper part of the underlying formation seems to have 
held originally much ferruginous as well as siliceous matter, 
